1. Field of the Invention
The present invention relates to a shadow mask for forming roughly rectangular beam spots on a fluorescent screen of a color cathode ray tube.
2. Background Art
A shadow mask 1 is mounted in a color cathode ray tube 101 with its surface facing to a fluorescent screen 102 of the color cathode ray tube 101, as shown in FIG. 8. In the color cathode ray tube 101, electron beams 105 emitted from electron guns 103, deflected by the magnetic fields produced by a deflection yoke 104, pass through the shadow mask 1 and then accurately strike predetermined points on the fluorescent screen 102.
The details of the shadow mask 1 are as follows. FIG. 4 is a diagrammatic plane view illustrating the positional relationship between the slots made in the shadow mask 1. As shown in this figure, the shadow mask 1 comprises a mask body 1a that is roughly rectangular in shape, and a large number of slots 2 (including slots 2a, 2b, 2c, and 2d) are arranged in this mask body 1a in the horizontal direction X and in the vertical direction Y in the plane view, each slot having a roughly rectangular through-hole that penetrates the mask body 1a in the direction of thickness. In this Specification, a unit structure composed of a through-hole, and a front-side opening and a backside opening that form the through-hole is referred to as a “slot”. Further, in FIG. 4, reference numeral 6 denotes a center (also referred to as a “center point”) that is the intersection of two diagonal axes 5, 5 connecting the opposite corners of the mask body 1a, extending along the mask body 1a plane; reference numeral 3, a horizontal axis passing through the center 6, extending along the mask body 1a plane; and reference numeral 4, a vertical axis passing through the center 6, extending along the mask body 1a plane. Furthermore, in FIG. 4, reference numeral 2a denotes a slot situated in the center 6 of the mask body 1a; reference numeral 2b, slots situated in the outer end part of the vertical axis 4; reference numeral 2c, slots situated in the outer end part of the horizontal axis 3; and reference numeral 2d, slots situated in the outer end part of each diagonal axis 5. FIG. 4 is merely a diagrammatic view, and the slots shown in this figure are dimensionally exaggerated.
When such a shadow mask 1 is placed in the color cathode ray tube 101 shown in FIG. 8 with the surface of the shadow mask 1 facing to the fluorescent screen 102 of the color cathode ray tube 101, the electron beams 105 emitted from the electron guns 103 vertically enter the slot 2a situated in the center of the shadow mask 1 but obliquely enter, at angles θ, the slots 2b, 2c, and 2d that are situated in the outer end parts of the respective axes (the horizontal axis 3, the vertical axis 4, and the diagonal axes 5), that is, in the peripheral part of the shadow mask 1. For this reason, in the shadow mask 1, the positions of the front-side opening and the backside opening that form a slot are adjusted according to the position of the slot in the mask body.
FIGS. 5A, 5B, 5C and 5D are diagrammatical plane views showing the shape of the slots 2 (slots 2a, 2b, 2c and 2d) made in the respective parts of the mask body 1a of the shadow mask 1. In these figures, reference numeral 11 denotes through-holes of the slots 2. The through-holes 11 are made so that they connect the front-side openings 12 and the backside openings 13 that are etched in a thin metal sheet. The backside openings 13 are made on the side on which electron beams 7 are incident, and the front-side openings 12 are made on the side from which the electron beams 7 emerge. The backside openings 13 and the front-side openings 12 are made roughly rectangular in shape, and the front-side openings 12 are made large in area so that they do not obstruct the passage of the electron beams 7.
Since electron beams enter, from the front, the slot 2a situated in the center of the mask body 1a, the through-hole 11 (the backside opening 13) of this slot is made so that it is positioned almost in the center of the front-side opening 12, as shown in FIG. 5A. FIG. 5B shows the slot 2b situated in the outer end part of the vertical axis 4; FIG. 5C, the slot 2c situated in the outer end part of the horizontal axis 3; and FIG. 5D, the slot 2d situated in the outer end part of the diagonal axis 5. Electron beams 7 obliquely enter the slots 2b, 2c, and 2d that are situated in the peripheral part of the mask body 1a. Therefore, in order not to obstruct the passage of the electron beams 7 that have passed through the through-hole 11 of each slot, the front-side opening 12 is made so that its position is offset from the position of the through-hole 11 (the backside opening 13) to the peripheral part side in the mask body 1a. 
However, even when the offset arrangement as shown in FIG. 5 (such an arrangement that the position of the front-side opening 12 of a slot 2 is offset from the position of the through-hole 11 (the backside opening 13) according to the position of the slot 2 in the mask body 1a) is made, of the slots 2b, 2c and 2d made in the peripheral part of the mask body 1a, especially the slots 2d situated in the outer end part of each diagonal axis 5 have the shortcoming that, since electron beams 7 that have obliquely entered the slots 2d are partially blocked by the sidewalls of the front-side openings 12 and the backside openings 13 of these slots, the slots 2d cannot let the electron beams 7 strike the fluorescent screen of the cathode ray tube to form thereon beam spots in the desired shape.
In order to overcome this problem, shadow masks having such a structure that, of the two long sides of a roughly rectangular through-hole of each slot made in a mask body, the long side situated on the side apart from the center of the mask body has a protrudent part protruding in the direction opposite to the vertical axis of the mask body, from at least one of the upper and lower end parts of this long side, have been proposed in Japanese Laid-Open Patent Publications No. 320738/1989 and No. 6741/1993.
FIG. 6 is a plane view showing the shape of the slots in the conventional shadow mask described in Japanese Laid-Open Patent Publication No. 6741/1993. This figure shows the shape of the slot 2d situated in the upper-right outer end part of the diagonal axis 5 extending toward the upper right in the plane view of the mask body 1a (FIG. 4). This slot 2d has the following features: the position of the front-side opening 12 is offset from the position of the through-hole 11 (the backside opening 13) to the upper right, that is, to the peripheral part side, and, at the same time, of the two long sides of the through-hole 11, the long side situated on the right side, that is, on the peripheral part side, has a protrudent part 11a protruding toward the peripheral part side from the lower end part of this long side. Further, the front-side opening 12 of the slot 2d is made to have a roughly rectangular outline composed of a pair of left- and right-hand long sides 12a, 12b and a pair of upper and lower short sides 12c, 12d. 
Incidentally, cathode ray tubes have came to be made flat in recent years, like the flat-type color cathode ray tube shown in FIG. 8. In such a flat-type color cathode ray tube, therefore, the angles θ at which electron beams 7 enter the slots 2 made in the shadow mask 1, especially those slots 2 situated in the peripheral part of the shadow mask 1, have come to be significantly great, and such a phenomenon that electron beams 7 that have passed through the through-holes 11 of the slots 2 are partially blocked by the sidewalls of the front-side openings 12 of the slots 2 occurs. For example, as shown in FIG. 6, although an electron beam 7a that has passed through the lower part of the through-hole 11 of the slot 2d situated in the outer end part of the diagonal axis 5 emerges from the slot 2d without striking the front-side opening 12 of this slot, an electron beam 7b that has passed through the upper part of the through-hole 11 of the slot 2d is partially blocked by the sidewall of the front-side opening 12 of this slot at a site encircled by the dotted line 8.
FIG. 7 is a sectional view taken along line VII—VII in FIG. 6 for explaining the above-described phenomenon. As shown in this figure, the front-side opening 12 of the slot 2d is composed of sidewalls 14, 15, the backside opening 13 of the slot 2d is composed of sidewalls 16, 17, and the through-hole 11 connects the front-side opening 12 and the backside opening 13. As shown in FIG. 7, in such a slot 2d, when an electron beam 7b that has passed through the upper part of the through-hole 11 passes through the front-side opening 12, a part of this electron beam 7b strikes the peripheral-part-side sidewall 15 of the front-side opening 12 and is thus blocked by the sidewall 15 at a site encircled by the dotted line 8 shown in FIG. 6. Occurrence of this phenomenon causes the following problems: the electron beam 7b that has passed through the slot 2d gets defective; the luminance is lowered; and the electron beams 7 cannot strike a fluorescent screen of a cathode ray tube to form thereon roughly rectangular beam spots in the desired size and shape.